This application deals with the regulation of sterol and bile acid biosynthesis in the rat, rabbit and other laboratory animals. Specifically, it is proposed to investigate: (1) The in vivo regulation of bile acid biosynthesis by the circulating bile acid pool, by cholesterol and other sterols, and by drugs, hormones, and diet. (2) The activities of certain rate-limiting enzymes of sterol and bile acid biosynthesis as affected by the circulating bile acid pool, by cholesterol and other sterols, and by drugs, hormones and diet. (3) The role of nuclear and side-chain hydroxylations in the biosynthesis of cholic acid and chenodeoxycholic acid.